Assessing forest resilience: The effects of wildfire and drought on forest structure in northern California, USA

John K. Loverin and Weimin Xi, Biological and Health Sciences, Texas A&M University-Kingsville, Kingsville, TX, Haibin Su, Department of Physics and Geosciences, Texas A & M University-Kingsville, Kingsville, TX, Jianwei Zhang, Pacific Southwest Research Station, USDA Forest Service, Redding, CA

Background/Question/Methods
Historical fire suppression in the western United States has resulted in large, difficult to control wildfires when conditions are favorable for fires to spread. Fire suppression has greatly changed forest structure in California, including increasing the density of trees that are shade tolerant. This has reduced the resilience of these forests to disturbances. Understanding and quantifying the resilience of forests to disturbances is increasingly important for forest ecosystem management. One way to address this problem is to use fuel treatments such as thinning and prescribed burning. In this study, we investigated changes in forest structure in Lassen and Plumas National Forest of northern California. We examined what management techniques can be used to restore a healthy forest structure and increase the resilience of forests to drought, fires, and climate change. We determined how forest management affects fire regime and forest structure. To accomplish this, we used 20 years of field data (including Forest Inventory and Analysis data) and aerial photos. We defined and measured forest resilience using quantitative metrics as described in the literature. To analyze the data, we used the Forest Vegetation Simulator (FVS), a forest growth simulation model, and ArcFuels, which implements existing models (including FVS) in ArcMap.
Results/Conclusions
We found that most of the forest stands are under 160 years old, and the most common age ranges are under 10 years old and between 70-110 years old. Annual tree mortality varied among species groups, with willows having the greatest mortality and laurels having the least. Trees between 45-60 m in height had the lowest mortality, and mortality was similar for trees of other heights. Trees under 5 inches in diameter had the greatest mortality. Trees living under 600 m in elevation had the most mortality, and trees between 600-1200 m had the least mortality. 12.9% of trees in the Lassen and Plumas National Forests were standing dead trees, and 65.8% of dead trees were standing. Climate change is expected to increase background tree mortality rates, and drought and fire suppression led to high tree mortality in California between 2012 and 2016. Forest managers need to prepare for increasing drought and fire risk in the future by modifying forest structure to increase resilience to these disturbances, for example by reducing tree density. Understanding forest structure, forest resilience, and the factors that make trees vulnerable to mortality will allow managers to better plan fuel treatments for these forests.